[agileAlligator]

Exactly how I learned to touch type; only it was Discord not IRC. The more things change, the more they remain the same.

[namaria]

Same here. I learned touch typing to furiously debate online as well =)

MSN messenger in my case haha

[Izkata]

I guess a less usual one, compared to the other comments: For me it was StarCraft multiplayer, had to type fast so I could get back to controlling my units.

Because I was rapidly going back and forth between the keyboard and mouse, it resulted in an unusual style where I use the outline of the keyboard for hand placement, and attempting to use the home row slows me down significantly.

[Terr_]

Starseige: Tribes and Counter-Strike: Sometimes a short thing while the character is coasting through the air.

Tribes in particular also had a voice-tree, and while it was shorter than prose it still encouraged a certain degree of touch-typing.

For example, typing VSAF (mnemonically [start][self][attack][flag]) led to text and prerecorded audio for "I will attack the enemy flag." Little of it survives now beyond references like VGZ for "Shazbot!"

[Esus]

Surprised no one has mentioned Runescape. Touch typing felt like the only option for GE spamming lol.